In recent years, titanium alloy (the alloy being allowable to be pure titanium; in the present specification, “titanium alloy” is a metal including, as an example thereof, pure titanium hereinafter) has come to be used in the field of aircrafts and various other fields. Under such a situation, titanium alloy manufacturers have paid attention to a technique of making use of, for example, an inexpensive titanium material or titanium scrap material large in unevenness of the shapes of pieces thereof, and unevenness of the composition thereof to manufacture titanium ingots low in costs and high in quality and reliability.
However, in a titanium ingot produced by melting, as titanium alloy, an inexpensive titanium material or titanium scrap material as described above, which is large in unevenness of the piece shape/composition thereof, the following remain: low density inclusions (hereinafter referred to as “LDIs”) having a specific gravity equivalent to or lower than that of titanium, specifically, a specific gravity of 5 g/cm3 or less; and high density inclusions (hereinafter referred to as “HDIs”) having a specific gravity more than that of titanium (specific gravity: more than 5 g/cm3). Thus, the inclusions produce a bad effect onto mechanical properties of the alloy. It is generally said that the proportion of the number of the LDIs as inclusions remaining in the titanium ingot to that of the LDIs as inclusions remaining in the titanium alloy as the raw material is from 5 to 6%. In the case of using titanium alloy, particularly, as a material for aircrafts, it is desired to make this proportion smaller. As a technique for solving such a problem, methods described below have been suggested.
Disclosed is, for example, a technique of an electron beam melting method using a hearth, in which an electron beam is scanned to a direction reverse to the direction along which titanium alloy melted in the hearth (hereinafter referred to as “melted titanium”) flows toward a mold, and further the average temperature of the melted titanium in the vicinity of a melted-titanium-outlet in the hearth is set to the melting point of LDIs therein or higher (see PTL 1). The use of this technique makes it possible to manufacture a titanium ingot in which the proportion of the LDIs is decreased from 5% to less than 1% by melting a raw material, i.e., a titanium sponge containing the LDIs, which have a grain diameter of 0.2 to 1.0 mm, together with HDIs, separating the HDIs by precipitation from the melted titanium, and further melting the LDIs in the melted titanium.
Disclosed is also a technique of causing the flow of melted titanium inside a hearth to rise along the vertical direction and subsequently descend, thereby making the residence period of the flow long to melt LDIs therein and further trap HDIs therein onto the bottom of the hearth (see PTL 2). The use of this technique makes it possible to manufacture a titanium ingot in which the proportion of the LDIs is decreased from 6% to less than 1% by melting a raw material, i.e., a titanium sponge containing the LDIs, which have a grain diameter of 1.0 to 3.0 mm, together with the HDIs, separating the HDIs by precipitation from the melted titanium, and further melting the LDIs in the melted titanium.